The invention relates to improvements in power trains which can be utilized with advantage in vehicles, especially in engine-driven motor vehicles. More particularly, the invention relates to improvements in methods of and in apparatus for transmitting torque in vehicular power trains. Still more particularly, the invention relates to improvements in methods of and in apparatus for transmitting torque from the rotary output element of a prime mover, such as a combustion engine in a motor vehicle, to a transmission (especially automatic transmission) having an input element which receives torque from the output element of the prime mover by way of a hydrokinetic torque converter and a slip clutch (known as lockup clutch or bypass clutch).
It is known to regulate the magnitude of torque which is being transmitted by a slip clutch, i.e., to vary the slip of the clutch which latter is normally connected in parallel with the hydrokinetic torque converter, by resorting to a monitoring system which serves to transmit signals denoting one or more variable parameters to a computerized central regulating or slip selecting unit.
In many power trains of the above outlined character, the hydrokinetic torque converter comprises a housing which receives torque from the output element of the prime mover and defines a fluid-containing chamber for a turbine and (if necessary or desired) a stator. The turbine is designed to transmit torque to the input element of the transmission. The slip clutch is confined in the housing and includes an axially movable piston (pressure plate) which divides the chamber into two fluid-containing compartments. The pressure differential between the bodies of hydraulic fluid in the two compartments of the chamber in the housing determines the extent of slip of the piston relative to an adjacent portion of the housing and hence the magnitude of torque which the piston of the engaged or partly engaged slip clutch can transmit from the housing (i.e., from the output element of the prime mover) to the turbine (i.e., to the input element of the transmission). The pump of the torque converter constitutes (or can constitute) a component part is of the housing. When the slip clutch is at least partially engaged, a friction face of the piston (or of a friction lining which is carried by the piston) is in more or less pronounced frictional engagement with a friction face of the adjacent portion of the housing of the torque converter (or with a friction face of a friction lining on the adjacent portion of the housing). Certain presently preferred configurations of the piston of the slip clutch and the adjacent portion of the housing of the torque converter, as well as certain presently preferred modes of preventing overheating of and excessive wear upon the friction faces and/or linings of the slip clutch are disclosed in the aforementioned and other commonly owned copending United States and other patent applications of the assignee of the present application.
German patent application No. 31 30 871 A1 (filed in the name of Takeo Hiramatsu and published Mar. 18, 1982) discloses a torque transmitting system wherein the torque regulating assembly comprises means for ascertaining the differences between the RPM of the output element of the prime mover and the RPM of the input element of the transmission receiving torque from torque converter. The thus ascertained differences are compared with desired values and the torque regulating assembly is designed to undertake the necessary steps if the ascertained actual differences depart from the desired or preselected values. The adjustment involves the establishment of a different pressure relationship between the bodies of hydraulic fluid in the compartments at opposite sides of the piston of the slip clutch, i.e., a change of the slip between the piston of the clutch and the adjacent portion of the torque converter housing. Such post-mortem regulation exhibits a number of serious drawbacks because a correction of the departure of actually ascertained unsatisfactory pressure differential from the desired or optimal pressure differential invariably takes place with a certain delay.
U.S. Pat. No. 5,029,087 (granted Jul. 2, 1991 to Ronald T. Cowan et al. for “Electronic control system for controlling torque converter bypass clutches”) discloses a system which also relies on the determination of differences between the actual slip of the lockup clutch and the desired slip to thereupon undertake the necessary corrective measures for the elimination or reduction of the ascertained differences. The adjustment of slip of the lockup clutch involves the establishment of a modified pressure differential between the bodies of hydraulic fluid at opposite sides of the axially movable piston or pressure plate of the lockup clutch. The drawbacks of such proposal are the same as those of the proposal in the published German patent application No. 31 30-871 A1 of Hiramatsu.
U.S. Pat. No. 4,577,737 (granted Mar. 25, 1986 to Yasuhiro Niikura et al. for “Lock-up torque converter and method for controlling clutch slip in lock-up torque converter”) discloses a further modification of the aforediscussed prior proposals. The patentees Niikura et al. employ a torque sensor for direct determination of the torque being transmitted by the hydrokinetic torque converter. The transmission of torque is selected in dependency upon the operating condition of the prime mover. This involves such selection of the slip of the lockup clutch that the magnitude of the actually transmitted torque matches the value dictated by the operating condition of the prime mover.
The proposal of Niikura et al. also involves a postmortem regulation of torque transmission from a prime mover to a transmission or another driven unit. Thus, it is again necessary to ascertain whether or not the magnitude of actually transmitted torque is satisfactory and to thereupon carry out necessary corrective undertakings if the monitored magnitude of torque departs from the desired or optimum magnitude. The only difference between the proposal of Niikura et al. and other previously mentioned conventional proposals is that Niikura et al. directly monitor the transmission of torque through the hydrodynamic drive.
The aforediscussed drawbacks of the proposals by Hiramatsu, Cowan et al. and Niikura et al. are believed to be the reason that such proposals failed to gain widespread acceptance in the relevant industries. Delayed adjustments to correct already existing unsatisfactory transmission of torque from a prime mover to a driven unit in the power train of a motor vehicle are particularly undesirable and deleterious in dynamic processes because the adjustments are not compatible with one or more aspects of a dynamic process.
A reduction of torque which is being transmitted by the rotary output element of a prime mover (such as a combustion engine in a motor vehicle) results in a reduction of slip of the lockup clutch which operates in parallel with a hydrokinetic torque converter in the power train between the prime mover and the transmission or another driven unit. In order to prevent blocking of the lockup clutch under the just outlined circumstances (i.e., in order to ensure that the lockup clutch can continue to prevent the transmission of fluctuations of torque from the prime mover to the driven unit), it is necessary to reduce the magnitude of torque which is being transmitted by the lockup clutch. In actual practice, the dynamics of variation of the magnitude of torque being transmitted by a lockup clutch are affected by delays and dead or idle times which are characteristic of the heretofore known torque regulating systems so that the lockup clutch is not likely to or cannot slip at all if the RPM of the output element of the prime mover which transmits torque to the slip clutch is less than 50 RPM.
Another reason that the aforedescribed conventional proposals to regulate the transmission of torque failed to gain acceptance in the automobile and related industries is that a vehicle is often put to use under circumstances when a time-dependent or time-optimized operation of the torque transmission regulating system is not desirable. The distribution of rotary masses in a motor vehicle is often such that the RPM of the input element of a gear shift transmission or an infinitely variable change-speed gearing (i.e., at the output element of the transmission) decreases when the transmission is shifted into a higher drive ratio or when the transmission ratio is changed but the RPM at the output of the transmission remains relatively constant. The slip of the lockup clutch increases in response to a reduction of the RPM of the output of the torque transmitting system which, however, in turn necessitates (due to the characteristics of the hydrokinetic torque converter) an increase of the slip of the lockup clutch. Such increase of slip necessitates an increase of the torque which is being applied to the input of the torque transmitting system, again due to the characteristics of the hydrokinetic torque converter. However, the driving unit is not in a condition to supply a higher torque under the just outlined circumstances of use of the torque transmitting apparatus in a motor vehicle. Therefore, the prime mover is braked to thus automatically establish a renewed slip at a lower level if the application of force to the lockup clutch remains unchanged during a change of the transmission ratio, such as shifting into a higher gear ratio. However, a torque regulator which is designed to carry out an adjustment at proper times will automatically attempt to counteract an increase of the slip of the lockup clutch by increasing the magnitude of the force acting upon the axially movable piston or pressure plate of the lockup clutch in a direction to enhance the frictional engagement between the friction face of the piston of the lockup clutch and the adjacent friction face of the cover or housing of the hydrokinetic torque converter. Therefore, the lockup clutch is likely to block when the torque regulating step is completed, i.e., the apparatus is then compelled to transmit fluctuations of torque from the output element of the prime mover (i.e., from the housing of the torque converter) to the turbine of the torque converter, i.e., to the input element of the transmission which receives torque from the torque converter and/or from the slip clutch.
German patent application No. 37 12 223 A1 of Noboru Sekine et al. (published Nov. 11, 1987) discloses a system which can regulate the transmission of torque in the power train of a motor vehicle, within certain speed ranges, in dependency on the position of the throttle valve and for the purpose of establishing a desired slip of the lockup clutch. The position of the throttle valve within the specific speed range influences the force which acts upon the piston of the lockup clutch in a sense to engage the clutch, i.e., to reduce the slip.
A drawback of the proposal of Sekine et al. is that the magnitude of torque which is being transmitted by the lockup clutch is a function of the position or setting of the throttle valve as well as a function of several other variable parameters including the friction coefficient of the friction lining(s) on the piston of the lockup clutch and/or on the adjacent portion of the housing of the hydrokinetic torque converter. The friction coefficient of a lining in a lockup clutch is dependent upon the variations of temperature, the extent of slip between the piston and the housing, the characteristics of the fluid (such as oil) which tills the housing of the torque converter and others. In other words, the friction coefficient is apt to, and often does, fluctuate within a very wide range. Therefore, the torque transmitting apparatus which is proposed by Sekine et al. also operates properly or acceptably only when the RPM of the housing of the torque converter and/or of the piston of the lockup clutch exceeds a certain minimum value. As a rule, the apparatus of Sakine et al. will fail, the same as at least some of the other aforediscussed apparatus, if the RPM of the output element of the prime mover is less than 50 and normally much more than 50. Such apparatus fail to ensure any savings or any appreciable savings in fuel consumption, i.e., the savings in fuel consumption are not more satisfactory than when the combustion engine of a motor vehicle drives a torque transmitting apparatus which does not embody a lockup clutch or bypass clutch.
Other known attempts to regulate the slip of a lockup clutch, even at 50 RPM and less, have also failed to meet the exacting standards in the automobile making and related industries. For example, it was proposed to provide a lockup clutch with a flat washer-like piston or pressure plate whose rigidity is low so that the magnitude of force which is required to urge the piston against the adjacent portion of the housing of the hydrokinetic torque converter cannot be selected and maintained with a requisite degree of precision and reproducibility. In other words, when the rigidity or stability of the piston is relatively low, the application of a force in a direction to engage such lockup clutch cannot ensure uniform distribution of pressures between the entire friction face of the piston and the entire friction face of the adjacent portion of the housing of the torque converter. This can result in partial overheating of the fluid (such as oil and known as ATF=automatic transmission fluid) which fills the housing of the torque converter and forms two at least partially separated fluid bodies at opposite sides of the piston. Attempts to overcome such drawbacks by increasing the area of the friction face of the piston and/or the adjacent portion of the housing have met with limited success because the space which is available for the installation of a torque transmitting apparatus in the power train of a motor vehicle is normally limited, especially as seen in the radial direction of the torque converter and the associated lockup clutch. Similar problems arise if the dimensions of the torque transmitting apparatus are to be increased in the axial direction of the torque converter. Attempts to employ one or more torsional dampers which are to absorb fluctuations of torque between the piston of the lockup clutch and the turbine of the torque converter and which are installed at a considerable radial distance from the axis of the torque converter are equally unsatisfactory, particularly in view of the large space requirements of such dampers. As a rule, dampers are used to counteract the aforediscussed drawbacks of all or nearly all conventional torque transmission regulating systems, namely that such systems are incapable of adequately regulating the slip of a lockup clutch at rotational speeds of the output element of the prime mover which are close to or less than fifty.